JP6969734B2 - Charge detection circuit and radiation detection device including it - Google Patents

Description

The present invention relates to a charge detection circuit that detects the amount of generated charge in time series and a radiation detection device including the same.

Conventionally, a mechanism of a detection circuit that detects a time-series charge amount for a current signal (charge signal) output from a radiation detector such as a semiconductor detector has been studied. For example, Non-Patent Document 1 below proposes a detection circuit that employs the ToT (Time over Threshold) method. This detection circuit can convert the current signal from the detector into a voltage signal with an amplifier, measure the time when the pulse of the voltage signal exceeds the threshold value, and output that time as the measurement result of the peak value of the pulse. can.

T.Orita, H. Takahashi and K. Shimazoe, “Dynamic Time-over-Threshold Method for Multi-Channel APD Based Gamma-Ray Detectors”, 2012 IEEE Nuclear Science Symposium and Medical Imaging Conference Record (NSS / MIC), 2012 , P. 824-826

However, in the detection circuit adopting the ToT method described above, the time width of the pulse of the voltage signal is converted into the crest value, but since the relationship between the time value and the crest value is not linear, the obtained crest value can be obtained. The accuracy of measurement results tends to be low. Further, in this detection circuit, a shaping circuit such as a filter circuit for shaping the pulse waveform of the voltage signal is required, so that the circuit configuration tends to be complicated.

The present invention has been made in view of the above problems, and is a charge detection circuit capable of accurately measuring the peak value of a charge signal that changes with time with a simple circuit configuration, and a radiation detection device including the same. The purpose is to provide.

In order to solve the above problems, the charge detection circuit according to one embodiment of the present invention is a circuit that detects the amount of generated charge in time series, includes a capacitor, receives an input of charge, and stores the charge in the capacitor. By doing so, an amplifier that converts a charge into a voltage signal, a comparator that generates a comparison signal by comparing the voltage signal and a predetermined threshold voltage at a predetermined time interval, and a comparator that generates a comparison signal at a predetermined time interval. When a counter that counts the duration of a predetermined state of the comparison signal and outputs a count value obtained by counting the duration of the predetermined state and a comparison signal generated by the comparer at a predetermined time interval indicates a predetermined state, the amplifier A charge extraction unit for sequentially extracting a predetermined amount of charge from the capacitor is provided.

Alternatively, the radiation detection device according to another embodiment of the present invention includes a charge detection circuit having the above configuration and a semiconductor detector that generates a charge in response to an incident of radiation and inputs the charge to the charge detection circuit.

According to the charge detection circuit of the above embodiment, a voltage signal is generated by accumulating the charge input in the amplifier, and a comparison signal is generated by periodically comparing the voltage signal with the threshold voltage in the comparator. Then, a predetermined amount of charge is extracted from the charge stored in the amplifier when the comparison signal indicates a predetermined state by the charge extraction unit. As a result, the signal of the input charge is converted into a continuous comparison signal converted in the time axis direction corresponding to the amount of the charge and output. According to the charge detection circuit having such a configuration, the peak value of the charge signal can be counted and output with high accuracy, and the shaping circuit is not required to shape the voltage waveform, so that the circuit configuration is simple. Will be transformed. As a result, it is possible to accurately measure the peak value of the charge signal that changes with time with a simple circuit configuration. Alternatively, according to the radiation detection device of the above-described embodiment, the device can be easily miniaturized, and a highly accurate radiation detection device can be realized.

Here, the counter counts the duration of a predetermined state indicating that the voltage signal exceeds the predetermined threshold voltage, and the charge extraction unit indicates that the comparison signal exceeds the predetermined threshold voltage. A predetermined amount of electric charge may be extracted when the predetermined state is indicated. In this case, the peak value of the signal of the input charge can be measured with high accuracy, and the circuit configuration of the charge extraction unit is simplified.

Further, the charge extraction unit may have a capacitor for accumulating a predetermined amount of charge in advance for extraction, or may have a constant current source for extracting a predetermined amount of charge. In this case, the circuit configuration of the charge extraction unit is easily simplified.

According to the present invention, it is possible to accurately measure the peak value of a charge signal that changes with time with a simple circuit configuration.

It is a block diagram which shows the schematic structure of the radiation detection apparatus which concerns on embodiment. It is a figure which shows the specific circuit structure of the charge extraction part of the charge detection circuit of FIG. It is a figure which shows the specific circuit structure of the charge extraction part of the charge detection circuit of FIG. It is a figure which shows the time waveform of various signals processed by the radiation detection apparatus of FIG. It is a block diagram which shows the schematic structure of the radiation detection apparatus which concerns on a comparative example.

Hereinafter, preferred embodiments of the charge detection circuit and the radiation detection device according to the present invention will be described in detail with reference to the drawings. In the description of the drawings, the same or corresponding parts are designated by the same reference numerals, and duplicate description will be omitted.

First, with reference to FIG. 1, the function and configuration of the radiation detection device 10 according to the embodiment of the radiation detection device of the present invention will be described. The radiation detection device 10 shown in FIG. 1 is a device for detecting the intensity of incident radiation that changes with time. The radiation detection device 10 is composed of a semiconductor detector 11 composed of one detection element and a charge detection circuit 12 for processing the output from the semiconductor detector, and has a two-dimensional distribution of the intensity of incident radiation. It may be composed of a plurality of semiconductor detectors 11 arranged two-dimensionally so as to be measurable, and a plurality of charge detection circuits 12 provided corresponding to each of the plurality of semiconductor detectors 11. Such a two-dimensional array circuit configuration can be realized by forming an integrated circuit on a semiconductor substrate.

As shown in FIG. 1, the radiation detection device 10 includes a semiconductor detector 11 and a charge detection circuit 12. The charge detection circuit 12 is a circuit that is electrically connected to the semiconductor detector 11 and receives a charge signal generated from the semiconductor detector 11 according to the intensity of incident radiation, and is a pre-amplifier (amplifier). It has 13, an analog-to-digital converter (ADC) 14, a counter 15, and a charge injection circuit (charge extraction unit) 16. Hereinafter, the configuration of each component of the radiation detection device 10 will be described.

The semiconductor detector 11 is a detector that generates electron-hole pairs (charge pairs) with the incident of radiation such as X-rays and converts the incident radiation into a current signal (charge signal) corresponding to the amount of energy thereof. .. Examples of such a detector include a Cd (Zn) Te semiconductor detector, a Si semiconductor detector, a Ge semiconductor detector, a GaAs semiconductor detector, a GaN semiconductor detector, a TlBr semiconductor detector, and the like.

The preamplifier 13 of the charge detection circuit 12 is a circuit unit that receives a charge as a current signal from the semiconductor detector 11 and converts the charge into a voltage signal by accumulating the charge. Specifically, the preamplifier 13 includes a capacitor, and by accumulating a charge in the capacitor, the voltage across the capacitor is output as a voltage signal.

The ADC 14 of the charge detection circuit 12 periodically compares a voltage signal with a predetermined threshold voltage at a predetermined time interval (for example, 100 nsec interval) by oversampling processing, and outputs a comparison signal generated as a result of the comparison. It is a department. That is, the ADC 14 is a comparator that compares a voltage signal and a threshold voltage, outputs a high-level comparison signal when the voltage signal exceeds the threshold voltage, and outputs a high-level comparison signal when the voltage signal is equal to or lower than the threshold voltage. Outputs a low level comparator signal. That is, the ADC 14 is a 1-bit analog-to-digital (A / D) converter.

The charge injection circuit 16 of the charge detection circuit 12 sequentially extracts a predetermined amount of charge from the capacitor in the preamplifier 13 when the comparison signal periodically generated by the ADC 14 at a predetermined time interval indicates a predetermined state. It is a circuit part for. The charge injection circuit 16 indicates that the comparison signal output from the ADC 14 is in a high level state, and when the voltage signal exceeds the threshold voltage in the operation of the immediately preceding ADC 14, the capacitor is connected to the semiconductor detector 11. By supplying a charge having the opposite polarity to the supplied charge, a predetermined amount of charge is extracted from the capacitor. The charge injection circuit 16 performs this charge extraction operation in synchronization with the periodic comparison operation by the ADC 14, and the voltage signal is the threshold value until the comparison signal becomes low level, that is, in the operation of the ADC 14 immediately before by the comparison signal. Continue until it is shown to be below the voltage.

The counter 15 of the charge detection circuit 12 is a circuit unit that counts the duration of a predetermined state of the comparison signal periodically generated by the ADC 14 at a predetermined time interval, and generates and outputs the count signal obtained as a result. Is. That is, the duration of the high level state of the comparison signal output from the ADC 14 is counted (counted) in synchronization with the periodic comparison operation by the ADC 14. More specifically, the counter 15 adds 1 to the count value of the count signal when the comparison signal detects a high level state at a predetermined time interval which is an operation interval of the ADC 14. Then, the counter 15 outputs a signal indicating the count value at a predetermined time interval (for example, 1.6 μsec interval), thereby outputting a digital signal indicating the distribution of the amount of electric charge in the time axis direction. The counter 15 generates a counting signal while resetting the counting value at predetermined time intervals. For example, when the A / D conversion interval by the ADC 14 is 100 nsec intervals and the count value is output at 1.6 μsec intervals, a 4-bit count signal can be generated. As such a counter 15, a known counter circuit such as a flip-flop can be used.

FIG. 2 shows an example of the circuit configuration of the charge detection circuit 12 described above. In the configuration shown in FIG. 2, a bias is applied to one terminal of the semiconductor detector 11 and the other terminal of the semiconductor detector 11 is connected to the input of the preamplifier 13. Further, the preamplifier 13 is composed of a differential amplifier 13a and a capacitor 13b, the non-inverting input of the differential amplifier 13a is connected to the ground, and the inverting input of the differential amplifier 13a is connected to the semiconductor detector 11. A capacitor 13b is connected between the output of the dynamic amplifier 13a and the inverting input. With such a configuration, the electric charge input from the semiconductor detector 11 is accumulated in the capacitor 13b, and a voltage signal corresponding to the amount of the electric charge is generated in the output of the differential amplifier 13a. Further, the charge injection circuit 16 is a switched capacitor circuit, and is composed of a DC power supply 16a, a capacitor 16b, and switch elements 16c to 16f. The charge injection circuit 16 having such a configuration receives the comparison signal EN from the ADC 14 and the clock signal CLOCK synchronized with the periodic comparison operation by the ADC 14, and the comparison signal EN is at a high level at the timing synchronized with the clock signal CLOCK. At the time of indicating, the charge of the charge amount corresponding to the voltage of the DC power source 16a stored in advance in the capacitor 16b is supplied to the capacitor 13b of the pre-amplifier 13. At this time, the polarity of the DC power supply 16a is set so that the electric charge supplied from the capacitor 16b to the capacitor 13b has a polarity opposite to the polarity of the electric charge supplied from the semiconductor detector 11 to the capacitor 13b. .. Specifically, one end of the DC power supply 16a is connected to the ground, the other end of the DC power supply 16a is connected to one end of the capacitor 16b via the switch element 16c, and the other end of the capacitor 16b is connected to the capacitor via the switch element 16f. It is connected to the terminal on the semiconductor detector 11 side of 13b. Further, both ends of the capacitor 16b are connected to the ground via the switch elements 16d and 16e, respectively. In the charge injection circuit 16 having the above configuration, the switch element 16c and the switch element 16e are closed, and the switch elements 16d and 16f are opened, so that the charge is accumulated in the capacitor 16b in advance. After that, when the comparison signal EN shows a high level at the timing synchronized with the clock signal CLOCK, the switch element 16c and the switch element 16e are opened, and the switch elements 16d and 16f are closed, so that they are stored in the capacitor 16b. Charges are supplied to the capacitor 13b.

FIG. 3 shows an example of another circuit configuration of the charge detection circuit 12. In the configuration shown in FIG. 3, the charge injection circuit 16A is a switched current circuit, and is composed of a constant current source 16g and a switch element 16h. The charge injection circuit 16A having such a configuration charges a charge amount corresponding to the current value of the constant current source 16 g when the comparison signal EN shows a high level at the timing synchronized with the clock signal CLOCK. It is supplied to the capacitor 13b of. At this time, the polarity of the constant current source 16g is such that the electric charge supplied from the constant current source 16g to the capacitor 13b has the opposite polarity to the polarity of the electric charge supplied from the semiconductor detector 11 to the capacitor 13b. Set. Specifically, one end of the constant current source 16g is connected to the ground, and the other end of the constant current source 16g is connected to the terminal on the semiconductor detector 11 side of the capacitor 13b via the switch element 16h. In the charge injection circuit 16A having the above configuration, when the comparison signal EN shows a high level in one cycle of the clock signal CLOCK, the switch element 16h is closed, so that the current value of the constant current source 16g and the clock signal A charge amount determined by the product of the CLOCK periods is supplied to the capacitor 13b.

Next, an operation example of the above-mentioned radiation detection device 10 will be described while comparing with a comparative example.

FIG. 5 shows a schematic configuration of the radiation detection device 910 according to the comparative example. The radiation detection device 910 according to the comparative example includes a detection circuit adopting the ToT method, and has a waveform shaping circuit 915 and a time-to-digital converter (TDC) 914 connected to the subsequent stage of the preamplifier 13. There is. The waveform shaping circuit 915 is a circuit for reducing the noise of the voltage signal output from the preamplifier 13 and shaping and outputting the voltage signal, and has a built-in filter circuit and an amplifier. This waveform shaping circuit 915 is required to improve the accuracy of T / D conversion by the TDC 914 in the subsequent stage. The TDC 914 compares the voltage signal shaped by the waveform shaping circuit 915 with a predetermined threshold voltage, counts the period during which the pulse exceeds the threshold voltage, and outputs the counting result as a digital value. In such a radiation detection device 910, the time width of the pulse is counted and output as representing the peak value of the pulse, but since the peak value and the time width do not have a linear relationship, the obtained peak value can be obtained. The accuracy of digital values tends to be low. Further, as a result of the waveform shaping circuit being indispensable for ensuring the accuracy of the output digital value to some extent, the circuit scale tends to be large. Furthermore, in order to increase the resolution of the output digital value, it is necessary to increase the number of threshold values in the TDC 914, and the circuit scale of the TDC 914 tends to increase.

FIG. 4 shows time waveforms of various signals processed by the radiation detection device 10 and the radiation detection device 910, (a) is a pulse waveform of an input current signal, and (b) is a radiation detection device 910. The waveform of the output voltage of the pre-amplifier 13 is supplied to (c) the waveform of the output voltage of the pre-amplifier 13 after the charge extraction operation in the radiation detection device 10, and (d) is supplied to the ADC 14 in the radiation detection device 10. The waveform of the clock signal CLOCK and the waveform of the output voltage (comparison signal) of the ADC 14 in the radiation detection device 10 are shown in (e). As shown in these time waveforms, a voltage signal having a level corresponding to the pulse of the input current signal is output from the preamplifier 13 of the radiation detection device 10, and the level of the voltage signal is the clock signal CLOCK. It is gradually decreasing at the timing synchronized with. Then, the output voltage of the ADC 14 of the radiation detection device 10 is maintained at a high level until the output level of the preamplifier 13 of the radiation detection device 10 drops to a predetermined level. On the other hand, as shown in the waveform of (b), the output voltage of the preamplifier 13 of the radiation detection device 910 of the comparative example is maintained at a constant voltage according to the pulse of the input current signal. This output voltage is processed by the waveform shaping circuit 915 and the TDC 914 in the subsequent stage.

According to the radiation detection device 10 described above, a voltage signal is generated by accumulating the electric charge input in the preamplifier 13, and the voltage signal is periodically compared with the threshold voltage in the ADC 14 for comparison. A predetermined amount of charge is withdrawn from the charge stored in the preamplifier 13 when the signal is generated and the comparison signal exhibits a high level state by the charge injection circuit 16. As a result, the signal of the input charge is converted into a continuous comparison signal converted in the time axis direction corresponding to the amount of the charge and output. Further, by counting the number of times the high level state of the comparison signal is sustained by the counter 15, it is possible to output a digital signal showing the distribution of the amount of electric charge in the time axis direction. According to the radiation detection device 10 having such a configuration, since the peak value of the current signal and the count value related to the high level state of the comparison signal are in a linear relationship, the peak value of the current signal is counted and output with high accuracy. be able to. Further, as compared with the radiation detection device 910 of the comparative example, a shaping circuit for shaping the voltage waveform is not required, and a 1-bit A / D converter is sufficient, so that the circuit configuration is simplified. As a result, it is possible to accurately measure the peak value of the charge signal that changes with time with a simple circuit configuration. Further, it is possible to realize a highly accurate radiation detection device with easy integration of the device.

Further, the charge injection circuit 16 adopts a switched capacitor configuration or a switched current circuit configuration. In this way, by configuring the structure to directly extract the charge instead of processing the digital signal, an extra circuit such as an operational amplifier becomes unnecessary, the circuit configuration of the charge injection circuit 16 is simplified, and the radiation detection device is used. Is easy to integrate.

The present invention is not limited to the above-described embodiment. The charge detection circuit 12 of the present embodiment is not limited to the use of radiation detection, and may be widely used as a detection circuit for an image pickup device. For example, it may be used for detecting γ-rays, α-rays, and the like.

10 ... Radiation detector, 11 ... Semiconductor detector, 12 ... Charge detection circuit, 13 ... Pre-amplifier (amplifier), 13b ... Capacitor, 14 ... ADC (Comparator), 15 ... Counter, 16, 16A ... Charge injection Circuit (charge extraction part), 16b ... Capacitor, 16g ... Constant current source.

Claims (3)

A circuit that detects the amount of electric charge generated in chronological order.
An amplifier that includes a capacitor and converts the charge into a voltage signal by receiving the input of the charge as a pulsed current signal and accumulating the charge in the capacitor.
A comparator that generates a comparison signal by comparing the voltage signal and a predetermined threshold voltage at a predetermined time interval, and a comparator.
A counting value obtained by counting the number of times the predetermined state of the comparison signal is generated by the comparator at a predetermined time interval is output as a value indicating the peak value of the charge input signal. ,
A charge extraction unit that sequentially extracts a predetermined amount of charge from the capacitor of the amplifier when the comparison signal generated by the comparator at a predetermined time interval indicates the predetermined state.
Charge detection circuit , including
A semiconductor detector that generates an electric charge in response to the incident of radiation and inputs the electric charge to the electric charge detection circuit.
Equipped with
The counter counts the number of durations of the predetermined state indicating that the voltage signal exceeds the predetermined threshold voltage.
The charge extraction unit extracts the predetermined amount of charge when the comparison signal indicates the predetermined state indicating that the voltage signal exceeds the predetermined threshold voltage, and the comparison signal draws the predetermined threshold value. Until it is shown that the voltage has dropped below the voltage, the predetermined amount of charge is repeatedly extracted in synchronization with the comparison operation of the comparator.
When the amplifier indicates that the comparison signal is equal to or lower than the predetermined threshold voltage, the amplifier receives the charge from the semiconductor detector in a state where the supply of the charge by the charge extraction unit to the capacitor is stopped. Accumulated in the capacitor
Radiation detector . The charge extraction unit has a capacitor for accumulating the predetermined amount of charge in advance for extraction.
The radiation detection device according to claim 1 . The charge extraction unit has a constant current source for extracting the predetermined amount of charge.
The radiation detection device according to claim 1 .

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